This invention relates to a bandmill having an automatic track and strain control system.
A bandmill is used for cutting lumber. A conventional vertical bandmill comprises a support frame, two wheels, one disposed vertically above the other, and an endless saw blade trained about the wheels. The lower wheel is driven and the upper wheel idles. The blade is maintained under tension, and accordingly when the lower wheel is driven the blade passes endlessly about the wheels. A cutting throat is provided between the upper and lower wheels along the downward run of the blade. In order to ensure that the blade remains in position on the wheels, the wheels are crowned and their relative positions are accurately determined so that the plane containing the maximum circumference of the upper wheel coincides with the plane containing the maximum circumference of the lower wheel. This need for accurate positioning of the wheels implies that the conventional bandmill is expensive to construct because a massive support structure is required in order to support the wheels with the required degree of stability.
The support structure that is conventionally used for a bandmill comprises a concrete base and a support frame mounted on the base. The support frame includes a mechanism for adjusting the vertical position of the upper wheel, whereby the tension in the blade can be adjusted. However, if the upper wheel is moved, it is then necessary to readjust the relative positions of the wheels to achieve precise coincidence of the planes containing the maximum circumference of the two wheels.
Sometimes, it is desirable to employ a bandmill in which the blade passes through the cutting throat in the horizontal direction, or at an angle that is inclined to the horizontal. However, the support structure of the vertical bandmill does not permit ready adjustment of the orientation of the bandmill. The nature of the support structure also implies that the orientation in which a given bandmill will be used is fixed at the time of manufacture, and accordingly it is necessary to build distinct bandmills for vertical, horizontal and inclined use.
The support structure for the conventional bandmill is not only massive but is also bulky. If two conventional bandmills are disposed in the same orientation and on the same side of the cutting path, they can not be any closer together than about two feet. It is proposed in co-pending application Ser. No. 07/089,489 filed Aug. 21, 1987, that a sawmill should be provided in which any three bandmills out of a group of four bandmills act on a single log on a single pass of the log through a bandmill station. The four bandmills are stationary with respect to the direction of feed of the log through the group of bandmills. The sawmill is designed to process logs that are only eight feet long. The cutting edges of the saw blades must therefore be closer together than about two feet, since otherwise the sawing by the upstream bandmill will be completed before the log starts to be sawn by the downstream bandmill, and this may create difficulties in log handling.
During sawing, the saw blade of a conventional bandmill vibrates in the cutting throat. If the blade is dull or the feed rate is high, the blade tends to snake out of the desired cutting plane as a log is fed through the throat. Consequently, lumber with rough surfaces or of uneven thickness is produced. Moreover, snaking may result in the blade being deflected to such an extent that it strikes other parts of the bandmill, resulting in damage to the bandmill. Hitherto, these problems have been addressed by using a thicker blade, which results in greater kerf loss, or increasing the blade tension, which results in higher maintenance costs.
When the blade of a bandmill is deflected, e.g., due to nonuniformities in the log being cut, there is a tendency for blade oscillations to occur, resulting in thicker kerf and impaired accuracy.
The stress distribution in the blade of a bandmill depends on the path followed by the blade relative to the wheels, and the stress distribution has a bearing on sawing accuracy. It has been found that improved accuracy is obtained if the bottoms of the gullets of the teeth are maintained close to the edges of the wheels, so that the stiffness of the blade in the teeth area is maximized. However, if the gullets pass over the wheels, there is a tendency for the saw blade to crack in the vicinity of the gullets.